The present invention relates, in general, to an overvoltage protection circuit. More particularly, the present invention relates to an overvoltage protection circuit of especial utility in providing secondary protection from transient current surges to an input/output line, in particular, in telephone applications.
Numerous applications require overvoltage protection circuits. U.S. Pat. No. 4,321,644 issued to Brasfield for "Power Line Transient Limiter" issued Mar. 23, 1982 describes a transient limiter for suppressing high voltage transients induced on an alternating current carrying power line in aircraft applications. U.S. Pat. No. 4,306,185 issued to Leuschner for a "Breakdown Voltage Protection Circuit" issued Dec. 15, 1981 discloses a high voltage breakdown protection circuit for limiting the voltage applied to a junction of an MOS circuit.
In telephonic applications in particular, transients can appear on the ring and tip lines to various telephone equipment due to their location adjacent power lines on telephone poles. These power lines can induce longitudinal, or common mode, signals on the telephone lines which can also be induced by the crosstalk of adjacent telephone lines. In addition, transients can appear on the ring and tip lines due to near lightning misses, environmental static discharge, faulty equipment or by vandalism. Primary overvoltage protection due to lightning, etc., is provided by either a carbon arc block, a gas discharge tube, spark gap or other means for providing protection between 300 to 800 volts. However, at lower voltage levels, there exists a need to allow voltages on the telephone line to exceed the power supply voltages without forward biasing an overvoltage protection network and to maintain overvoltage protection at a reasonable voltage level and pass all surge current to system ground or earth potential. To this end, U.S. Pat. No. 4,323,942 issued to Hartman et al for a "Solid State Protector Circuitry Using Gated Diode Switch" issued Apr. 6, 1982 describes a circuit to facilitate the rapid discharge of high voltage transients, in particular on telephone lines. However, the circuitry therein described is incapable of returning a transient appearing on a telephone line to ground or earth potential. U.S. Pat. No. 4,377,832 issued to Toney et al for a "Voltage Transient Suppressor Circuit" issued Mar. 22, 1983 describes a circuit for use with telephonic systems in which negative voltage transients are shorted to earth potential such that the telephone equipment, for example an integrated circuit subscriber loop interface circuit (SLIC), are not damaged by negative high voltage transients. The circuit disclosed in the foregoing patent, while a decided improvement over a conventional diode bridge circuit, nonetheless requires connection to the -48 volt battery supply located at the telephone central office.
It is therefore an object of the present invention to provide an improved overvoltage protection circuit.
It is further an object of the present invention to provide an improved overvoltage protection circuit which does not require connection to the central office power source.
It is still further an object of the present invention to provide an improved overvoltage protection circuit which is of particular utility in telephonic applications.
It is still further an object of the present invention to provide an improved overvoltage protection circuit which is readily and inexpensively fabricated utilizing a minimum of components.
It is still further an object of the present invention to provide an improved overvoltage protection circuit which can provide overvoltage protection for surge currents of any predetermined value.